1. Field of the Invention
The present invention relates to a system, a method, and a circuit for distance measurement between two nodes of a radio network.
2. Description of the Background Art
In a radio network, it is desirable to locate the nodes of the radio network or to determine at least one distance between the nodes. As a result, for example, a defective node can be easily found. Slow movements of nodes, for example, a means of conveyance in a factory, can also be tracked. A locating of the nodes can also be used advantageously in firefighting, when the nodes dropped by an airplane can be located and a locally increased temperature transmitted.
U.S. Pat. No. 5,220,332 discloses a distance measuring system which has an interrogator and a transponder and enables the nonsimultaneous measurement between two objects. A carrier signal is modulated with a (low-frequency) modulation signal with a variable modulation frequency to determine by a phase measurement or alternatively by a transit time measurement a distance between the interrogator and the transponder from the change in the modulation signal.
International Pat. Appl. No. WO 02/01247 A2 discloses a method for measuring the distance between two objects with the use of electromagnetic waves. An interrogation signal of a base station and a response signal of a portable code emitter are transmitted twice at different carrier frequencies. The carrier frequencies in this case are correlated; i.e., they are dependent on one another. The carrier frequencies are approximated to one another, so that a phase shift between the signals can be measured. The distance of the code emitter to the base station is calculated from this phase shift. The interrogation signal and the response signal can be transmitted at different carrier frequencies or at the same carrier frequencies. The carrier frequencies are altered for a renewed interrogation/response dialog.
If a transceiver of a node for a sensor network is laid out according to the industry standard 802.15.4 for a half-duplex system, it cannot transmit and receive simultaneously. If said transceiver is to be used as an active reflector for phase measurement, the node therefore must store the phase of the received signal, for example, by a phase-locked loop and after switching from receiving to transmitting again use the same stored phase for transmitting. For example, during reception by an additional phase-locked loop, the crystal oscillator of the transceiver of the node functioning as the active reflector is adjusted so that the frequency and phase of the LO signal (LO—Local Oscillator) of the local oscillator match the receive signal. During switching to transmission, the additional phase-locked loop must be opened and the crystal oscillator now synchronized in frequency runs freely further. As a result, the transceiver of the node functioning as the active reflector transmits with the same or proportional phase position and with the same frequency, as it previously had received a carrier signal. In this respect, very high requirements are placed on a free-running oscillator with regard to frequency stability and phase stability. Disturbances must be avoided, such as, for example, crosstalk of signals in the integrated circuit, which can cause phase changes.
U.S. Pat. No. 6,731,908 B2 discloses a method for determining the distance between two objects for Bluetooth technology. In this case, the frequency is changed by frequency hops to measure a phase offset for multiple different frequencies. An object has a voltage-controlled crystal oscillator in a phase-locked loop (PLL), whereby the phase-locked loop is closed during the receiving and opened during the transmission, so that the receive signal and transmit signal have the same frequency. The phase of the local oscillator signal of the voltage-controlled crystal oscillator due to the synchronization by the PLL is thereby coherent to the received signal.